Process for the oxidation of organic compounds



United States Patent 3,155,718 PRGCESS FGR TEE OXHDATION 0F QRGANHCUMPOUNDS Alhyn William Norman Brown, Colin Bertie (Iotteriil, Fred Dean,and Gordon Howard Whitfield, all of Norton-on-Tees, England, assignorsto imperial Chernical Industries Limited, London, England, a corporationof Great Britain No Drawing. Filed 9st. 28, 1957, Ser. No. 692,534Claims priority, application Great Britain Nov. 21, 1956 4 Claims. (Cl.26tl-524) This invention relates to the oxidation of organic compounds.

According to the invention there is provided a process for the oxidationof organic compounds in the liquid phase by means of molecular oxygen orozone characterised in that the oxidation is conducted with condensationof the overhead vapours, separation of water from the condensate, andreflux, under such conditions that the composition of the vapour overthe liquid is in each stage of the oxidation outside the explosive rangeof compositions. Desired product can be recovered from the reactionproduct, for example by distillation.

The oxidation may be conducted in a single stage or in several stages.Moreover, the process may be operated batchwise or continuously. It isdesirable that cooling be provided in the process, for example by meansof the latent heat of vaporization of a reactant or reactants or of aninert solvent, or by the use of a coil through which fluid iscirculated, or by external heat transfer means. The last mentioned isespecially important when the reaction mixture under the oxidationconditions atfords little self-cooling by evaporation of some of itscomponents. The oxidation zone is preferably furnished with externalheat transfer means, for example a jacket containing a liquid, forexample water, boiling under pressure, and an associated condenser; thestarting material is introduced near the top, and the oxidising gas nearthe bottom of the reaction zone; and the product is taken oil at or nearthe bottom thereof. Preferably, also, the uncondensed vapours from thecondenser are scrubbed with a liquid for recovery of residual startingmaterial they contain.

In processes comprising several stages of oxidation each of the stagesmay be furnished with the auxiliaries just described. Further, inprocesses comprising more than one stage any or all of the stages maycomprise two or more sub-stages each similarly conducted.

Where there are several oxidation zones distillation columns arepreferably provided between them and can be run either, (i) to yield asoverheads unoxidised start ing materials and intermediate oxidationproducts for return to the prior oxidation stage, and as bottomsoxidation products for feeding to the next oxidation stage; or (ii) toyield as overheads unoxidised starting materials for return to the prioroxidation step and as bottoms intermediate and final oxidation productsfor feeding to the next oxidation step. 7

The process can be operated firstly under such conditions that theconcentration of organic compounds in the vapour over the liquid isbelow that corresponding to the lower explosive limit. According to thismethod of operation it may be arranged that the vapour in equilibriumwith the liquid in the oxidation reactor has a concentration of organiccompounds below that corresponding to the lower explosive limits; or tooperate with a liquid, the equilibrium vapour of which would lie withinthe explosive limits, but to bring the composition of the vapour belowthe lower explosive limit by introducing sufiicient incombustible, inertdiluent into the liquid or into the space above the liquid in thereaction zone.

3,155,718 Patented Nov. 3, 1964 Suitable incombustible inert diluentsare, for example, nitrogen, carbon monoxide and steam.

The process can be operated secondly under such conditions that theconcentration of organic compounds in the vapour over the liquid isabove that corresponding to the upper explosive limit. According to thismethod of operation it may be arranged that the vapour in equilibriumwith the liquid in the oxidation reactor has a concentration of organiccompounds above that corresponding to the upper explosive limit, or tooperate with a liquid of which the equilibrium vapour would lie withinthe explosive limits but to bring the composition of the vapour abovethe upper explosive limit by introducing sufiicient volatile organicliquid either into the liquid or into the space above the liquid, sothat in result the concentration of organic compounds in the actualvapour above the liquid is above that corresponding to the upperexplosive limit. Sometimes this method of operation can be conducted byincreasing the rate of boiling. It is often convenient to introduce thevolatile organic liquid as a stream of vapour above the liquid. Suitableorganic liquids are, for example, benzene, acetic and propionic acids,halogenated benzenes, e.g., chlorbenzene and orthodichlorbenzene, andfully halogenated aliphatic hydrocarbons such as carbon tetrachloride.

Another advantage of using such volatile organic liquids is that theyfunction also to remove water formed in the reaction either asazeotrope, or by direct contribution of partial pressure. This water canbe removed, for example, by means of a separator such as a Dean andStark decanter situated in the exit line from the still of the reactor.

When operating so that the vapour over the liquid has a concentration oforganic compounds above the upper explosive limit, it is advantageous tocirculate the vapour in a closed system comprising a condenser and apurge line: the condensed starting material, and volatile solvent if oneis used, are recycled to the oxidation step and the gas is purged. Asimilar procedure may be adopted, when a gaseous inert diluent is used,the condensed starting material being returned to the oxidation step andalso some of the inert gas if this is economically justifiable.

It is an important feature of the invention that the oxidising gas bedistributed highly efficiently in the reaction mixture: this appliesespecially when operating above the upper explosive limit. Suitabledistributors are, for example, rapidly rotating gas mixers, e.g., acruciform stirrer, high efliciency atomisers, turbulent jets, or sinterglass: of these the first two are preferred. By using such distributorsthe oxygen content of the vapour above the liquid can be kept to aminimum.

Increase in total pressure most frequently favours high utilization ofoxygen and high reaction velocity in the reaction stages.

Most frequently in continuous operation counter-current flow of liquidand oxidising gas is preferred because then the highest concentration ofoxidising gas is brought into contact with the material containing mostoxidation product. However, if desired, co-current flow of liquid andoxidising gas may be used.

In many oxidations it is preferred to use substantially pure oxygen asthe oxidising gas, because the over-all velocity of the reaction islargely determined by the rate of diifusion of the gas. It is thenparticularly important that a highly efficient gas distributor be used,as already described. However, in fast oxidation processes, or when thesolubility of oxygen in the liquid is high, air or oxygen diluted withinert gases may conveniently be used. If substantially pure oxygen isused for these oxidations it is economic to recycle the exit gas streamfrom the oxidation reactor after removing oxides of carbon, for

3 example with aqueous alkali; by water extraction, or aqueoussolutions.

It is an important feature of the present invention to operate theprocess outside the explosive range of compositions. These limits may bedetermined for a given mixture by the method described at pages 9 and 10of Bulletin 503 of the USA. Bureau of Mines, entitled, Limits ofFlammability of Gases and Vapours. The term limits of flammabilitynormally refers to ignition at atmospheric pressure. Where ignitionoccurs in a closed system ignition is of the nature of an explosion andthus determination of the flammability limit is a suflicient index ofthe explosive limits (see, for example, Principles of ChemicalEngineering, by Walker, Lewis and Mc- Adams (McGraw-Hill), SecondEdition, Seventh Impression, page 206, second paragraph).

For less volatile liquids the correct mixture to be tested can beprepared by passing a stream of the oxidising gas through a series oftowers each containing an adsorbent such as kieselguhr loaded with theorganic liquid. The towers are maintained at that temperature whichyields a mixture with a partial pressure of organic compoundcorresponding to that which will be present in the reactor of theoxidation process. Care is taken to avoid condensation before feedingthe mixture to the explosion tube by providing suitable heating means.The temperature in the explosion tube can be varied to correspond to theactual temperature in the oxidation process by providing the explosiontube with an external heating coil.

As discussed at page 3 of Bulletin 503, pressure has some modifyingeffect on the explosive limits and this must be borne in mind and dueallowance made for it when the oxidation process is operated atsuperatmospheric pressure. For practical operation it is sufficient tomake a determination of the concentrations of an organic compound ormixture of compounds corresponding to the lower and upper explosivelimits by the methods above referred to, and then to arrange theoperating conditions of the process so that the concentration is welloutside these limits. Thus it may be necessary to work 1% or 2% or evenfurther outside the explosive limits, but the width of tolerancenecessary is a matter well within the skill of the qualified technicalchemist. In many cases the question simply is: will a given mixtureexplode or not? A test made as above described will quickly decide this.

In those stages of the process where operation is above the upperexplosive limit of the vapour, the concentration of the organic compoundwill very often after condensation, which is a necessary step, fallwithin the explosive range. This may happen, due to inefliciency ofcondensation, even when the equilibrium mixture of the gas and organiccompound would not normally be explosive at the temperature in thecondenser. It is therefore preferred to make provision for the admissionof sufiicient inert diluent, e.g., nitrogen, carbon dioxide or otherinert gas, in close proximity to the potential hazard in order toeliminate it, for example in the exit gas line after the condenser.

According to another method of operation, which is continuous, theozone, oxygen or oxygen-containing gas, is brought into contact with theorganic compound in the liquid phase under such conditions that theconcentration of organic compounds in the vapour above the liquid isgreater than corresponds to the upper explosive limit, at least part ofthe oxidise liquid is taken off continuously and fed to a separatingcolumn; the overhead from this comprising the starting material isrecycled to the reactor and the bottoms comprising intermediate andfinal oxidation products are fed to a second reactor provided with acondenser, reflux and decanter, in which they are brought into contactwith air or oxygen or ozone, run under such conditions that theconcentration of organic compounds in the vapour over the liquid is lessthan that corresponding to the lower explosive limit. The vapour fromthe second reactor emerging from the condenser may be subjected in theexit gas line to further cooling or scrubbing for recovery of valuableresidual compounds, if this is economic. As an alternative to the aboveprocedure one or all of the intermediate products may be returned asoverheads from the column to the first stage for oxidation there. inthis process it is sometimes desirable to have present in the stageoperated above the upper explosive limit, a more volatile combustibleoxidation-stable solvent as already described.

In another form of the invention, which is continuous, the ozone, oxygenor oxygen-containing gas, is brought into contact with the organiccompound in the liquid phase and partial oxidation is effected undersuch conditions that the concentration of organic compounds in thevapour above the liquid is greater than the upper explosive limits, atleast a portion of the liquid mixture is withdrawn continuously to asecond reactor in which the oxidation is continued and the compositionof the vapour above the liquid is controlled outside the explosivelimits by circulating it together with an inert in a closed systemincluding a condenser and a purge line, and the liquid product from thesecond reactor is introduced into a third reactor in which it is reactedwith oxygen or air under such conditions that the concentration oforganic compound in the vapour is below the explosive limit. Bothreactors are provided with reflux stills and with Dean and Starkdecanters in the exit lines.

According to another continuous method applicable to some compounds itis possible to operate a two stage oxidation using two reactors in thefirst of which the concentration of the organic compound in the vapourabove the liquid is above the explosive limit and in the second of whichthe liquid product from the first stage is oxidised under conditionssuch that the concentration of the organic compound above the liquid isbelow the explosive limit, each reactor being fitted with a refluxcolumn provided with a Dean and Stark decanter in the exit line. Thenthe second stage is preferably operated at a higher total pressure.

The process can also be operated by distributing the organic compound tobe oxidised in the oxidising gas by means of a packed tower or anatomizer nozzle, so that the liquid forms films.

The process is of especial value in relation to the oxidation of organiccompounds in the liquid phase by means of ozone or molecular oxygen inthe presence of a catalyst comprising a metal of variable valence, e.g.,manganese and/or cobalt and bromine, commonly under superatmosphericpressure.

Example 1 60 grams of ortho-xylene dissolved in grams of molten benzoicacid and containing as catalyst 0.33 gram of MnBr .4H O and 0.19 gram ofCoBr .6H O was oxidized in the liquid phase at C. and atmosphericpressure by passing oxygen into it at a rate of 24 litres per hourthrough a high speed cruciform stirrer. Simultaneously 300 litres perhour of steam measured at 100 C. was blown over the liquid reactionsurface. The vessel was provided with a reflux column and a Dean andStark decanter in the gas exit line, and water was condensed from theexit gas stream. Analysis of the uncondense gas showed average oxygenabsorption of approximately 3 litres per hour. 20 mls. of benzene wereincluded in the reaction mixture with the object of avoiding blocking ofthe decanter by benzoic acid carried over, and this, with anyunconverted ortho-xylene was continuously returned to the oxidationvessel by overflow.

After 14 hours running 27 mls. of water was added to the reactionmixture, and the latter was hydrolysed for 1% hours under reflux. Onfiltering the molten mixture, boiling the precipitate with ether,refiltering and drying the precipitate, there was obtained 14.7 grams ofphthalic acid (99.2% pure).

Example 2 40 grams of para-xylene dissolved in 200 grams of propionicacid was treated with well dispersed oxygen at 137 C. under reflux inthe presence as catalyst of 0.33 gram of MnBr .4H O and 0.19 gram ofCoBr .6H O, using an oxygen rate of 12 litres per hour. The boil-up ratewas maintained at approximately 2 gram moles per hour of propionic acidto ensure that the concentration of the latter in the vapour was suchthat the vapour was above the higher inflammability limit. The vapoursfrom the reaction vessel passed through a packed reflux column and werecondensed in a decanter in the gas exit line. The bottom phase,consisting of para-xylene dissolved in propionic acid, was returned tothe oxidation reactor.

After 20 hours, the process was stopped, the reaction medium wasfiltered, and there was obtained 56.8 grams of almost colourlessterephthalic acid (97% pure), which corresponds to a molar pass yield of90.6%.

Example 3 The proportions and procedural details were the same as inExample 2 except that the boil-up rate was decreased to 0.5 gram molepropionic acid per hour. 100 litres of propionic acid vapour (measuredat 150 C.) was blown over the surface of the reaction medium at 150 C.in order to maintain the total composition of the vapour outside itsupper infiammability limit.

After 20 hours the process was stopped and on filtration of thesuspension there was obtained 56.7 grams of terephthalic acid (95.3%pure), which corresponds to a molar pass yield of 90.5%.

Example 4 40 grams of ortho-xylene dissolved in 200 grams of moltenbenzoic acid containing as catalyst 0.33 gram of and 0.19 gram of CoBr.6H- O together with 50 grams of benzene, was oxidised with a welldispersed stream of oxygen at 150 C. under atmosphereic pressure using12 litres per hour of oxygen. The boil-up rate of the liquid wasmaintained at about 300 ml. of liquid hydrocarbon per hour, and thecomposition of the vapour thus maintained above its upper infiammabilitylimit. The hydrocarbon vapours from the reaction zone after passagethrough a packed reflux column were condensed in a decanter in the gasexit line, the lower aqueous phase was removed from the system, and thehydrocarbon was recycled to the oxidation reactor.

After running for 20 hours, 27 mls. of water was added to the mixture,and the whole was hydrolysed under total reflux for 2 hours. Onfiltration of the molten reaction medium there was obtained crudeortho-phthalic acid, which, after purification by boiling withortho-xylene,

7 yielded on drying, 9.3 grams of substantially pure acid.

The amount of ortho-xylene in the filtrate was again made up to 40 gramsand the process was repeated. This was repeated another four times, andthere was obtained in all 258.5 grams of ortho-phthalic acid (98.3%pure). 22 grams of ortho-xylene was recovered from the cold trap, andtherefore the overall yield was 91.2% molar.

Example 5 The proportions of the reactants and the procedural detailswere the same as in Example 4 except that the boil-up rate of the liquidwas decreased to 50 mls. of hydrocarbon per hour. 50 litres per hour ofindustrial nitrogen measured at 20 C. was blown over the reactionsurface, whereby the upper inflammability limit was lowered, so thatunder these conditions the vapour over the liquid was non-inflammable.

After 20 hours the process was stopped and following treatment in amanner similar to that described in Example 4, 13.2 grams of phthalicacid was obtained for one pass.

Example 6 For this run 40 grams of ortho-xylene and 0.5 gram of CoBr .6HO were added to the filtrate obtained after filtering oil the phthalicacid from the product obtained in Example 5. The other proceduraldetails were the same, except that carbon dioxide at a rate of 50 litresper hour, measured at 20 C., was passed over the surface of the liquid.

After 20 hours, the process was stopped, the product was hydrolysed byadding 18 mls. of water and refluxing for 1 /2 hours, and theortho-phthalic acid precipitated was filtered ofr. 50.8 grams ofsubstantially pure orthophthalic acid were obtained.

We claim:

1. In processes for the production of benzene carboxylic acids byoxidation in the liquid phase at elevated temperatures with gaseousoxygen of the alkyl side-chains of alkyl-substituted benzenehydrocarbons in the presence of a catalyst selected from the groupconsisting of cobalt bromide and manganese bromide and wherein there ispresent above said liquid phase a vapor phase containing hydrocarbon andoxygen, the improvement which consists essentially of the combination ofsteps of:

(1) incorporating a hydrocarbon solvent in said liquid phase, relativelymore volatile than any of the other components of said phase; and,

(2) maintaining a boil-up rate such that the vapor of said solventmaintains the hydrocarbon composition of the vapor phase over saidliquid phase above the upper explosive limit thereof.

2. The process of claim 1 wherein said solvent is selected from thegroup consisting of volatile benzene hydrocarbons, lower aliphaticacids, halogenated benzenes, and fully halogenated aliphatichydrocarbons.

3. In processes for the production of benzene carboxylic acids byoxidation in the liquid phase at elevated temperatures with gaseousoxygen of the alkyl side-chains of alkyl-substituted benzenehydrocarbons in the presence of a catalyst selected from the groupconsisting of cobalt bromide and manganese bromide and wherein there ispresent above said liquid phase a vapor phase containing hydrocarbon andoxygen, the improvement which consists essentially of the combination ofsteps of:

(l) employing a boil-up rate no more than normal for said liquid phase;while,

(2) introducing vapors of a volatile organic liquid into the vapor phaseover the surface of said liquid phase so that the hydrocarboncomposition of the vapor phase is maintained above its upper explosivelimit.

4. The process of claim 3 wherein said volatile organic liquid isselected from the group consisting of volatile benzene hydrocarbons,lower aliphatic acids, halogenated benzenes, and fully halogenatedaliphatic hydrocarbons.

References Cited in the file of this patent UNITED STATES PATENTS2,444,924 Farkas et al. July 13, 1948 2,552,278 Hochwalt May 8, 19512,673,217 Hull Mar. 23, 1954 2,680,757 Himel June 8, 1954 2,761,872 HannSept. 4, 1956 2,788,367 Bill et al. Apr. 9, 1957 2,833,816 Satfer et al.May 6, 1958 2,890,245 Bonnet June 9, 1959

1. IN PROCESSES FOR THE PRODUCTION OF BENZENE CARBOXYLIC ACIDS BYOXIDATION IN THE LIQUID PHASE AT ELEVATED TEMPERATURES WITH GASEOUSOXYGEN OF THE ALKYL SIDE-CHAINS OF ALKYL-SUBSTITUTED BENZENEHYDROCARBONS IN THE PRESENCE OF A CATALYST SELECTED FROM THE GROUPCONSISTING OF COBALT BROMIDE AND MANGANESE BROMIDE AND WHEREIN THERE ISPRESENT ABOVE SAID LIQUID PHASE A VAPOR PHASE CONTAINING HYDROCARBON ANDOXYGEN, THE IMPROVEMENT WHICH CONSISTS ESSENTIALLY OF THE COMBINATION OFSTEPS OF: (1) IN CORPORATING A HYDROCARBON SOLVENT IN SAID LIQUID PHASE,RELATIVELY MORE VOLATILE THAN ANY OF THE OTHER COMPONENTS OF SAID PHASE;AND, (2) MAINTAINING A BOIL-UP RATE SUCH THAT THE VAPOR OF SAID SOLVENTMAINTAINS THE HYDROCARBON COMPOSITION OF THE VAPOR PHASE OVER SAIDLIQUID PHASE ABOVE THE UPPER EXPLOSIVE LIMIT THEREOF.